distance from MTB chainring to frame.

Hello, I am new to this forum. I currenty have a 98 Trek Y3, and a 2001 (I think) Gary Fisher Joshua F3. My dilemna is I am building an aluminum rearend for the Y3. I purchased a Phil Wood titanium BB for this new rearend. My question is what is supposed to be the distance between the smallest of three chainrings to the frame?

What is a good way to measure chainline to seat tube?

I have a set of calipers and is difficult to measure correctly, do you guys have a trick up your sleeves for that? Any way, from what I could measure, I got little over 53mm. So will try to adjust the BB spindle tommorrow. But if I shoot for perfect, aim for 48.5mm? Or is this more something when the rear deraileur and hub with cassette is all together to determine chainline? Trek was kind to leave an indented dot that I assume is center to seat tube.

I have a set of calipers and is difficult to measure correctly, do you guys have a trick up your sleeves for that? Any way, from what I could measure, I got little over 53mm. So will try to adjust the BB spindle tommorrow. But if I shoot for perfect, aim for 48.5mm? Or is this more something when the rear deraileur and hub with cassette is all together to determine chainline? Trek was kind to leave an indented dot that I assume is center to seat tube.

A ruler can work well. Somewhat depends on the seat tube thickness as well as your rear gear setup; 47.5 was common for the thinner seat tubes, moved out to 50 with the fatter ones for the most part. Try this http://sheldonbrown.com/chainline.html